Lipopolysaccharides and outer membrane proteins as main structures involved in complement evasion strategies of non-typhoidal Salmonella strains.

Non-typhoidal Salmonella (NTS) infections pose a serious public health problem. In addition to the typical course of salmonellosis, an infection with Salmonella bacteria can often lead to parenteral infections and sepsis, which are particularly dangerous for children, the elderly and immunocompromised. Bacterial resistance to serum is a key virulence factor for the development of systemic infections. Salmonella, as an enterobacterial pathogen, has developed several mechanisms to escape and block the antibacterial effects of the complement system. In this review, we discuss the relevance of outer membrane polysaccharides to the complement evasion mechanisms of NTS strains. These include the influence of the overall length and density of the lipopolysaccharide molecules, modifications of the O-antigen lipopolysaccharide composition and the role of capsular polysaccharides in opsonization and protection of the outer membrane from the lytic action of complement. Additionally, we discuss specific outer membrane protein complement evasion mechanisms, such as recruitment of complement regulatory proteins, blocking assembly of late complement components to form the membrane attack complex and the proteolytic cleavage of complement proteins.

Functional assessment of rat complement pathway activities and quantification of soluble C5b-9 in an experimental model of renal ischemia/reperfusion injury.

There is a growing interest in the monitoring of complement activation, not only in clinical settings but also in experimental models. However, for rodents only a limited number of tools are available to assess complement activity and activation. Here we describe three ELISAs for the measurement of rat classical (CP), MB-lectin (LP) and alternative (AP) pathway activities in serum and plasma. Moreover, we optimised a soluble C5b-9 (sC5b-9) ELISA for the detection of low level complement activation in rat. We determined the conditions for correct sample handling and showed that the assays had low inter- and intra-assay variation. We applied these assays to monitor complement activation in an experimental rat model of renal ischemia/reperfusion injury. We did not observe major complement consumption following reperfusion in CP or LP, and only minor AP consumption at 24h post reperfusion. However, MBL depletion prior to ischemia/reperfusion using a monoclonal antibody, transiently and specifically inhibited 75% of LP activity and ameliorated the AP consumption at 24h. To further assess complement activation during renal IRI, we monitored serum sC5b-9 and found that it was only significantly increased 72h post-reperfusion, but not when rats were pre-treated with anti-MBL or after sham surgery. In conclusion the described assays enable sensitive, reproducible and comprehensive assessment of complement activation in experimental rat models.